A trap or trapped antenna is configured to be resonant at multiple frequencies. To achieve this mode of operation, the trap antenna contains a number of electrically isolated sections, which are separated by multiple traps. Each of the traps includes a parallel resonant circuit that is tuned to a different resonant or operating frequency. Because the parallel resonant circuit of each of the traps is tuned to a different resonant frequency, the trap antenna is able to exhibit the correct feed-point impedance when transmitting and receiving communication signals at those various resonant frequencies.
Due to their design, trap antennas are unable to be easily tuned to receive or transmit new or additional radio communication frequencies that have come into use after the manufacture of the trap antenna. In fact, in order to retune the trap antenna to work acceptably with a new or additional communication frequency, an additional trap would have to be inserted between the existing traps of the trap antenna. Such a modification would require the redesign of the circuitry of the original traps so that they continue operating at their original resonant frequencies.
Traditional methods of adding frequencies to non-trapped antennas involve adding an additional coil and conductive section at a particular point on the original antenna that resonates at the desired frequency and provides the correct impedance at the feed-point of the antenna at the additional resonant frequency. However, utilizing such a technique with an antenna containing traps alters the resonant frequency established by the parallel tuned circuit of the trap to which the tuning circuit is attached, which is unwanted.
Thus, there is a need for a tuning circuit for a trap antenna that may be easily attached to an existing trap antenna. Moreover, there is a need for a tuning circuit for a trap antenna that enables an existing trap antenna to operate at a new operating frequency without affecting the resonant frequency of the trap and the antenna to which the tuning circuit is attached.